Hoi Tang Ma

Synchronization of HeLa cells.

HeLa is one of the oldest and most commonly used cell lines in biomedical research. Owing to the ease of which they can be effectively synchronized by various methods, HeLa cells have been used extensively for studies of the cell cycle. Here we describe several protocols for synchronization of HeLa cells from different phases of the cell cycle. Synchronization in G(1) phase can be achieved with the HMG-CoA reductase inhibitor lovastatin, S phase with a double thymidine block procedure, and G(2) phase with the CDK inhibitor RO3306. Cells can also be enriched in mitosis by treating with nocodazole and mechanical shake-off. Release of the cells from these blocks enables researchers to follow gene expression and other events through the cell cycle. We also describe several protocols, including flow cytometry, BrdU labeling, immunoblotting, and time-lapse microscopy, for validating the synchrony of the cells and monitoring the progression of the cell cycle after release.

Cyclin A2―Cyclin-Dependent Kinase 2 Cooperates with the PLK1―SCFβ-TrCP1―EMI1―Anaphase-Promoting Complex/Cyclosome Axis To Promote Genome Reduplication in the Absence of Mitosis

ABSTRACT Limiting genome replication to once per cell cycle is vital for maintaining genome stability. Inhibition of cyclin-dependent kinase 1 (CDK1) with the specific inhibitor RO3306 is sufficient to trigger multiple rounds of genome reduplication. We demonstrated that although anaphase-promoting complex/cyclosome (APC/C) remained inactive during the initial G2 arrest, it was activated upon prolonged inhibition of CDK1. Using cellular biosensors and live-cell imaging, we provide direct evidence that genome reduplication was associated with oscillation of APC/C activity and nuclear-cytoplasmic shuttling of CDC6 even in the absence of mitosis at the single-cell level. Genome reduplication was abolished by ectopic expression of EMI1 or depletion of CDC20 or CDH1, suggesting the critical role of the EMI1-APC/C axis. In support of this, degradation of EMI1 itself and genome reduplication were delayed after downregulation of PLK1 and β-TrCP1. In the absence of CDK1 activity, activation of APC/C and genome reduplication was dependent on cyclin A2 and CDK2. Genome reduplication was then promoted by a combination of APC/C-dependent destruction of geminin (thus releasing CDT1), accumulation of cyclin E2-CDK2, and CDC6. Collectively, these results underscore the crucial role of cyclin A2-CDK2 in regulating the PLK1-SCFβ-TrCP1-EMI1-APC/C axis and CDC6 to trigger genome reduplication after the activity of CDK1 is suppressed.

CDK1 inhibitors antagonize the immediate apoptosis triggered by spindle disruption but promote apoptosis following the subsequent rereplication and abnormal mitosis.

Spindle-disrupting agents and CDK inhibitors are important cancer therapeutic agents. Spindle toxins activate the spindle-assembly checkpoint and lead to sustained activation of CDK1. Different published results indicate that CDK1 activity is either important or dispensable for the cytotoxicity associated with spindle disruption. Using live cell imaging and various approaches that uncoupled mitotic events, we show that apoptosis was induced by both prolonged nocodazole treatment as well as by inhibition of CDK1 activity after a transient nocodazole block. However, distinct mechanisms are involved in the two types of cell death. The massive apoptosis triggered by nocodazole treatment requires the continue activation of cyclin B1-CDK1 and is antagonized by premature mitotic slippage. By contrast, apoptosis induced by nocodazole followed by CDK inhibitors occurred after rereplication and multipolar mitosis of the subsequent cell cycle. The presence of dual mechanisms of cytotoxicity mediated by spindle disruption and CDK inhibition may reconcile the various apparent inconsistent published results. These data underscore the essential role of cyclin B1-CDK1 as the basis of apoptosis during mitotic arrest, and the role of mitotic slippage and abnormal mitosis for apoptosis at later stages.

p53 deficiency enhances mitotic arrest and slippage induced by pharmacological inhibition of Aurora kinases

A number of small-molecule inhibitors of Aurora kinases have been developed and are undergoing clinical trials for anti-cancer therapies. Different Aurora kinases, however, behave as very different targets: while inhibition of Aurora A (AURKA) induces a delay in mitotic exit, inhibition of Aurora B (AURKB) triggers mitotic slippage. Furthermore, while it is evident that p53 is regulated by Aurora kinase-dependent phosphorylation, how p53 may in turn regulate Aurora kinases remains mysterious. To address these issues, isogenic p53-containing and -negative cells were exposed to classic inhibitors that target both AURKA and AURKB (Alisertib and ZM447439), as well as to new generation of inhibitors that target AURKA (MK-5108), AURKB (Barasertib) individually. The fate of individual cells was then tracked with time-lapse microscopy. Remarkably, loss of p53, either by gene disruption or small interfering RNA-mediated depletion, sensitized cells to inhibition of both AURKA and AURKB, promoting mitotic arrest and slippage respectively. As the p53-dependent post-mitotic checkpoint is also important for preventing genome reduplication after mitotic slippage, these studies indicate that the loss of p53 in cancer cells represents a major opportunity for anti-cancer drugs targeting the Aurora kinases.

Determinants of Mitotic Catastrophe on Abrogation of the G2 DNA Damage Checkpoint by UCN-01

Genotoxic stress such as ionizing radiation halts entry into mitosis by activation of the G2 DNA damage checkpoint. The CHK1 inhibitor 7-hydroxystaurosporine (UCN-01) can bypass the checkpoint and induce unscheduled mitosis in irradiated cells. Precisely, how cells behave following checkpoint abrogation remains to be defined. In this study, we tracked the fates of individual cells after checkpoint abrogation, focusing in particular on whether they undergo mitotic catastrophe. Surprisingly, while a subset of UCN-01–treated cells were immediately eliminated during the first mitosis after checkpoint abrogation, about half remained viable and progressed into G1. Both the delay of mitotic entry and the level of mitotic catastrophe were dependent on the dose of radiation. Although the level of mitotic catastrophe was specific for different cell lines, it could be promoted by extending the mitosis. In supporting this idea, weakening of the spindle-assembly checkpoint, by either depleting MAD2 or overexpressing the MAD2-binding protein p31comet, suppressed mitotic catastrophe. Conversely, delaying of mitotic exit by depleting either p31comet or CDC20 tipped the balance toward mitotic catastrophe. These results underscore the interplay between the level of DNA damage and the effectiveness of the spindle-assembly checkpoint in determining whether checkpoint-abrogated cells are eliminated during mitosis. Mol Cancer Ther; 10(5); 784–94. ©2011 AACR.

Timeless Interacts with PARP-1 to Promote Homologous Recombination Repair

Human Timeless helps stabilize replication forks during normal DNA replication and plays a critical role in activation of the S phase checkpoint and proper establishment of sister chromatid cohesion. However, it remains elusive whether Timeless is involved in the repair of damaged DNA. Here, we identify that Timeless physically interacts with PARP-1 independent of poly(ADP-ribosyl)ation. We present high-resolution crystal structures of Timeless PAB (PARP-1-binding domain) in free form and in complex with PARP-1 catalytic domain. Interestingly, Timeless PAB domain specifically recognizes PARP-1, but not PARP-2 or PARP-3. Timeless-PARP-1 interaction does not interfere with PARP-1 enzymatic activity. We demonstrate that rapid and transient accumulation of Timeless at laser-induced DNA damage sites requires PARP-1, but not poly(ADP-ribosyl)ation and that Timeless is co-trapped with PARP-1 at DNA lesions upon PARP inhibition. Furthermore, we show that Timeless and PARP-1 interaction is required for efficient homologous recombination repair.

Orderly Inactivation of the Key Checkpoint Protein Mitotic Arrest Deficient 2 (MAD2) during Mitotic Progression

Anaphase is promoted by the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) only when all the chromosomes have achieved bipolar attachment to the mitotic spindles. Unattached kinetochores or the absence of tension between the paired kinetochores activates a surveillance mechanism termed the spindle-assembly checkpoint. A fundamental principle of the checkpoint is the activation of mitotic arrest deficient 2 (MAD2). MAD2 then forms a diffusible complex called mitotic checkpoint complex (designated as MAD2(MCC)) before it is recruited to APC/C (designated as MAD2(APC/C)). Large gaps in our knowledge remain on how MAD2 is inactivated after the checkpoint is satisfied. In this study, we have investigated the regulation of MAD2-containing complexes during mitotic progression. Using selective immunoprecipitation of checkpoint components and gel filtration chromatography, we found that MAD2(MCC) and MAD2(APC/C) were regulated very differently during mitotic exit. Temporally, MAD2(MCC) was broken down ahead of MAD2(APC/C). The inactivation of the two complexes also displayed different requirements of proteolysis; although APC/C and proteasome activities were dispensable for MAD2(MCC) inactivation, they are required for MAD2(APC/C) inactivation. In fact, the degradation of CDC20 is inextricably linked to the breakdown of MAD2(APC/C). These data extended our understanding of the checkpoint complexes during checkpoint silencing.

Inhibitory Phosphorylation of Cyclin-Dependent Kinase 1 as a Compensatory Mechanism for Mitosis Exit

ABSTRACT The current paradigm states that exit from mitosis is triggered by the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) acting in concert with an activator called CDC20. While this has been well established for a number of systems, the evidence of a critical role of CDC20 in somatic cells is not unequivocal. In this study, we reexamined whether mitotic exit can occur properly after CDC20 is depleted. Using single-cell analysis, we found that CDC20 depletion with small interfering RNAs (siRNAs) significantly impaired the degradation of APC/C substrates and delayed mitotic exit in various cancer cell lines. The recruitment of cyclin B1 to the core APC/C was defective after CDC20 downregulation. Nevertheless, CDC20-depleted cells were still able to complete mitosis, albeit requiring twice the normal time. Intriguingly, a high level of cyclin-dependent kinase 1 (CDK1)-inhibitory phosphorylation was induced during mitotic exit in CDC20-depleted cells. The expression of an siRNA-resistant CDC20 rescued both the mitotic exit delay and the CDK1-inhibitory phosphorylation. Moreover, the expression of a nonphosphorylatable CDK1 mutant or the downregulation of WEE1 and MYT1 abolished mitotic exit in CDC20-depleted cells. These findings indicate that, in the absence of sufficient APC/C activity, an alternative mechanism that utilized the classic inhibitory phosphorylation of CDK1 could mediate mitotic exit.

Tetraploidization increases sensitivity to Aurora B kinase inhibition.

Aurora kinases are overexpressed in many cancers and are targets for anticancer drugs. The yeast homolog of Aurora B kinase, IPL1, was found to be a ploidy-specific lethality gene. Given that polyploidization is a common feature of many cancers, we hypothesized polyploidization also sensitizes mammalian cells to inhibition of Aurora kinases. Using two models of apparent diploid vs. tetraploid cell lines (one based on the hepatocellular carcinoma cell line Hep3B and another on untransformed mouse fibroblasts), we found that tetraploid cells were more sensitive to Aurora B inhibition than their diploid counterparts. Apoptosis could be induced in tetraploid cells by two different Aurora B inhibitors. Furthermore, tetraploid cells were sensitive to Aurora B inhibition but were not affected by Aurora A inhibition. Interestingly, the underlying mechanism was due to mitotic slippage and the subsequent excessive genome reduplication. In support of this, abolition of cytokinesis with dihydrocytochalasin B resulted in similar effects on tetraploid cells as Aurora B inhibition. These results indicate that inhibition of Aurora B or cytokinesis can promote apoptosis effectively in polyploid cancer cells.

Depletion of p31comet Protein Promotes Sensitivity to Antimitotic Drugs

Background: Slippage promotes premature mitotic exit during mitotic block. Results: Expression of p31comet regulates mitotic slippage and sensitivity to spindle poisons. Conclusion: Inactivation of p31comet delays mitotic slippage and increases cytotoxicity of spindle poisons. Significance: p31comet can be a target for antimitotic therapies. Antimitotic spindle poisons are among the most important chemotherapeutic agents available. However, precocious mitotic exit by mitotic slippage limits the cytotoxicity of spindle poisons. The MAD2-binding protein p31comet is implicated in silencing the spindle assembly checkpoint after all kinetochores are attached to spindles. In this study, we report that the levels of p31comet and MAD2 in different cell lines are closely linked with susceptibility to mitotic slippage. Down-regulation of p31comet increased the sensitivity of multiple cancer cell lines to spindle poisons, including nocodazole, vincristine, and Taxol. In the absence of p31comet, lower concentrations of spindle poisons were required to induce mitotic block. The delay in checkpoint silencing was induced by an accumulation of mitotic checkpoint complexes. The increase in the duration of mitotic block after p31comet depletion resulted in a dramatic increase in mitotic cell death upon challenge with spindle poisons. Significantly, cells that are normally prone to mitotic slippage and resistant to spindle disruption-mediated mitotic death were also sensitized after p31comet depletion. These results highlight the importance of p31comet in checkpoint silencing and its potential as a target for antimitotic therapies.